In the case where an AC voltage is applied to an AC rotating machine by use of an electric-power converter for converting a DC voltage into an AC voltage, the upper limit of the amplitude of the voltage applied by the electric-power converter to the AC rotating machine is determined by the amplitude of the DC voltage. In the case where the electric-power converter controls the voltages to be applied to the AC rotating machine based on the voltage commands on the rotation two axes, the induction voltage in the AC rotating machine increases in proportion to the rotation speed of the AC rotating machine and hence it is also required to enlarge the voltage commands on the rotation two axes; however, when reaching the upper limit of the voltage amplitude determined by the amplitude of the DC voltage, the amplitude of the voltage to be applied to the AC rotating machine cannot be made larger than the upper limit. In contrast, the phase of the voltage to be applied to the AC rotating machine can arbitrarily be given.
For example, in a conventional control device for an AC rotating machine disclosed in Patent Document 1, an electric-power conversion system, in which the currents of two phases out of the output currents of three phases of which are detected, the detected currents are coordinate-converted into the components of two perpendicularly intersecting axes in a rotating coordinate system, a voltage command obtained by amplifying the difference between the current commands is coordinate-converted again so that voltage commands of three phases are calculated, and then the electric-power conversion system is controlled in such a way that the output voltage of the electric-power conversion system follows the voltage commands, includes a first limiting circuit that limits the absolute value of a d-axis-component difference amplification signal to a first limit value; a control value calculation circuit that calculates a q-axis voltage limit value vqm, based on a d-axis-component voltage command value Vd*, which is the output signal of the first limiting circuit, and a second limit value vm and in accordance with the relationship “Vqm=(Vm2−Vd*2)1/2”; and a second limit value circuit that limits the absolute value of the d-axis-component difference signal to a limit value vqm, which is calculated by the control value calculation circuit. In addition, the output signal of the second limit value circuit is utilized as a q-axis-component voltage command Vq*. By configuring the control device in such a way as described above, control is performed in such a way that when the rotation speed of the AC rotating machine rises and the amplitude of the voltage outputted by the electric-power converter reaches the second limit value Vm, the amplitude of the voltage applied to the AC rotating machine is held at the second limit value Vm and the phase of the voltage applied to the AC rotating machine is shifted by tan−1 (Vqm/Vd*) from the d axis.
A conventional control device for an AC rotating machine disclosed in Patent Document 2 performs control in such a way that the output voltage command value of an electric-power converter that drives a permanent-magnet motor based on d-axis and q-axis current command values, d-axis and q-axis current detection values, a frequency calculation value, and a motor constant setting value, and in such a way that when the output voltage value of the electric-power converter is limited, a command value for the phase error, which is the difference between the control reference axis and the motor magnetic flux axis, is created based on the difference between the q-axis current command value and the q-axis current detection value. By configuring the control device in such a way as described above, control is performed in such a way that when the rotation speed of the AC rotating machine rises and the amplitude of the voltage outputted by the electric-power converter reaches an upper limit value, the difference between the q-axis current command value and the q-axis current detection value increases and a command for the phase error, which is the difference between the control reference axis and the motor magnetic flux axis, is given based on the difference so that the phase of the voltage applied to the AC rotating machine is shifted.